This invention relates to seismic detection apparatus and to a method of analyzing data acquired by such apparatus.
In seismic detection, a seismic source signal propagates through different rock substrates or strata within the earth, so producing compression wave (P-waves) and shear wave (S-waves) energy which can be analyzed to determine the direction and extent of geological features in strata.
Generally a seismic source signal is produced, either on land, sea, or in a borehole, the source signal usually being produced as either acoustic or elastic energy. Some of the energy radiates downward through geological layers within the earth and is reflected in varying proportions at different layer boundaries and this reflected energy can be detected at the surface. Some of the energy within the seismic source signal propagates directly along the ground producing a wave signal known as ground-roll. To detect the acoustic reflections which are in the form of P-waves and S-waves, a linear or areal array of geophones is often used to reinforce the reflected energy and to attenuate the energy associated with ground-roll so that a good signal to noise ratio can be achieved for the reflected wave components of interest.
It is desirable to be able to separate the P-wave and S-wave components of an elastic wave field as the separated components can be used more effectively to identify different characteristics than when combined. It is an aim of the present invention to provide improved separation of seismic signals into P-wave and S-waves.
Methods have been described which use three or more sensors in close proximity (e.g. less than 1 meter) for determining certain ground characteristics very close to the sensors. For example, in published European Patent Application 0 455 091 A2 (Application No. 91106459.0), a method is described which uses relatively closely spaced xe2x80x9coscillation sensorsxe2x80x9d to determine the xe2x80x9claminar structure and other characteristics of [the] ground.xe2x80x9d However, this method only provides information about characteristics of the ground within a distance from the sensor array of the order of the maximum wavelength of the surface waves (hereinafter referred to as the xe2x80x9cnear-site ground structurexe2x80x9d). Moreover, the method relies on sensing xe2x80x9cmicroseismsxe2x80x9d which is believed to refer to naturally occurring seismic activity. Thus, the published application does not disclose a method using seismic energy from a man-made seismic source, nor does it teach a method of sensing the components used to process reflected waves. Rather the method is only concerned with ground-roll type waves and does not teach or suggest a method suitable for hydrocarbon exploration using seismic energy reflected from man-made seismic sources.
According to one aspect of the present invention, there is provided seismic detection apparatus comprising seismic detection means capable of detecting a plurality of seismic components over a selected volume.
Preferably the seismic detection means comprises a plurality of receivers spaced apart to enclose the selected volume. The receivers may include hydrophones, geophones, accelerometers or a combination of these, and preferably are adapted to detect seismic signals along three mutually orthogonal axes. Thus the receivers may be provided by three-component geophones.
Preferably the selected volume is formed by at least three receivers spaced within a plane and at least one receiver placed external to the plane, thereby to enclose the selected volume over which measurement of seismic signals occurs.
In accordance with the invention, the receivers can be spaced to form a tetrahedron with three receivers spaced to define a triangle within the plane and a fourth receiver placed external the plane but in line with the centroid of the triangle. Other configurations of spaced apart receivers may be used depending on the characteristics of the wavefield being measured, for example receivers may be spaced to form an octahedral volume, a cubic volume, or a spherical volume. Certain shaped volumes may be more appropriate for different uses, for example receivers spaced to form an octahedral volume may be more appropriate for use with permanent downhole sensors.
The seismic detection apparatus in accordance with the present invention is suitable for use on the ground surface or ocean bottom. When used on the ground surface, the enclosed volume can be achieved by placing one receiver further below the surface of the other receivers, for example by burying it beneath the surface. With marine and ocean bottom surveys, the volume may be defined by tethering the receivers at different water depths, for example by placing three receivers spaced apart at the same depth, and a fourth receiver at a different depth. The seismic detection apparatus according to the present invention is also of use in seismic detection in transition zones such as swamps and marshes, where again tethering of the receivers at different depths will define the enclosed volume.
The apparatus may also have all the receivers incorporated into one body for attaching to a wireline and using downhole, or for using permanently downhole. Such a permanent sensor is of particular advantage where monitoring of a reservoir or other geological feature over time is required. This may be appropriate when monitoring fluid flow occurring within a reservoir as permanent sensors can be used to detect natural seismicity resulting from fluid opening and closing micro-fissures within the substrates and producing micro-earthquakes. Whilst these micro-earthquakes produce very weak broadband signals, by use of permanent sensors, this activity can be detected and production adapted by remedial action such as acid to remove carbonate deposits, or fracturing rock with high pressure steam or water, so as to improve production efficiency.
Preferably the apparatus further comprises processing means which analyses detected seismic components from individual receivers to separate P-wave components from S-wave components. The processing means may be provided at substantially the same location as the receivers to allow for on-site processing, or remote from the receivers to allow off-site processing.
In surveys of the vertical component of seismic data, the receivers may be adapted to detect components along one axis and orientation of the receivers selected so as to permit processing of one-component data to identify P-wave components. Therefore the receivers may be provided by one-component geophones.
Preferably the spacing of the receivers is selected to be smaller than the wavelength of the detected seismic components. A preferred spacing is less than 1 m, more preferably between 0.05 m and 0.50 m, although larger spacings of up to a third of the shortest detected wavelength are possible, and thus the range may be between 1.5 and 15 m.
In use, the apparatus may be operated to detect seismic signals over a time period of the order of five seconds where P-wave components are of principal interest, with any marginal S-wave component detected at that time being used to produce improvements in the signal to noise ratio.
Where a detection time of the order 10 to 15 seconds is used, both P-wave and S-wave components will be detected within the seismic signals and may be separated for multi-component inversion yielding images of reservoirs and other geological formations, and full reservoir characterization.
In accordance with another aspect of the invention, there is provided a method of processing seismic data comprising acquiring seismic data relating to a wavefield over a selected volume of acquisition, and measuring the curl and divergence of the wavefield from the seismic data, to thereby identify seismic components within the seismic data.
As P-waves are curl free and S-waves divergence free, this allows the separate identification of P-wave and S-wave components. Up-going and down-going wavefield components in boreholes may also be identified from the seismic data. Attenuation of unwanted seismic components from data acquired downhole is also enabled, and noise sources such as ground roll, airwave and inter-sensor perturbations can be removed from the data by signal processing.
The method preferably further comprises averaging the curl and divergence over the selected volume of acquisition, and where appropriate acquiring seismic data over a selected period of time.
Additionally the method may further comprise defining the selected volume of acquisition to be small compared to the wavelength of seismic waves to be detected.
As described above, in a seismic acquisition at a free surface, the constraints imposed by the free surface enable the reduction of the acquisition volume to a acquisition surface where all receivers are located in a plane.
In another aspect of the invention, the representation theorem based filters used for removing multiples from Ocean Bottom Cable data as described in the Published UK Patent Application GB 2 333 364 A (Published 21 Jul. 1999) is modified to provide a method of separating the seismic data acquired with the planar receiver configuration above into up- and downgoing wavefield.
In a variant that is particularly suitable for hydrocarbon exploration seismic acquisitions at a free surface or fluid/solid boundary, the volume formed by the receivers can be reduced to a plane. Hence, three 3-component receivers can be used in combination with the free surface (or fluid/solid) condition constraints to advantageously determine approximations to the spatial and temporal derivatives local to the receivers (hereinafter the xe2x80x9cfull wavefieldxe2x80x9d). The receivers should be adapted to measure the seismic energy from a seismic source that has been reflected from one or more subterranean earth structures outside the near-site ground structure.
According to a further aspect of the invention, each of the receivers are one-component geophones and a fourth seismic receiver is located less than about 1 meter from the other three seismic receivers.
A method for hydrocarbon exploration is also provided that includes imparting seismic energy into the earth that travels through the earth and reflects off of one or more subterranean earth structures outside the near-site ground structure. At least three seismic receivers are positioned substantially in a plane at or near the earth surface, and the seismic receivers are spaced less than about 1 meter from each other to form a receiver cluster. The received seismic energy is stored and analyzed to determine characteristics of one or more hydrocarbon reservoirs.
Preferably, the curl and divergence of the wavefield is determined from the seismic data, thereby identifying seismic components within the seismic data.
Preferably, the P-wave and S-wave components are separately identified, the up-going and down-going wavefield components are identified from the seismic data, and unwanted seismic components from seismic data is attenuated.
Preferably, the curl and divergence is averaged over the selected plane or volume of acquisition, and the selected plane or volume of acquisition is defined to be small compared to the wavelength of seismic waves to be detected.
These and other features of the invention, preferred embodiments and variants thereof, possible applications and advantages will become appreciated and understood by those skilled in the art from the detailed description and drawings following hereinafter.